You say? And I thought I was an agreeable GM! ;)
A Greater Creation of Sin in an Angel that works is a Deadly Wound to that Angel. It does not guarantee that Cneph will cast him out of Heaven, or that anyone else will. It does not guarantee that he'll take up residence in Hell. It does not ensure that he'll love everything in the world, but the evil things the most.
But will it put the Aspect "On Fire" on the angel?
The Hubris Rule isn't really for cases where obviously the angel won't fall. The HG in Nobilis is pretty powerful; they have plenty of tools to stop that from happening, including the perfectly fair and reasonable ones you've just suggested.
And it isn't really for cases where the angel falling is totally dramatically appropriate, either.
It exists because there are such cases, you understand. It exists because I didn't want to rule out the possibility that someday down the line it'd be totally correct in your game for the Power of Sin to whip a Major Creation of Sin at an angel, and have the angel fall. Or for the Power of Factories to make an Angel Factory. Or for the Power of Gifts to give someone the power to be Harumaph or Cneph, but only one at a time.
But it's not actually for those cases.
It's for those cases where it's almost right. It's for those cases when it's so close to being sensible, so close to being the right thing to do, and yet so wrong. It's there for every case where you just know that your Power is going to do something they shouldn't, they oughtn't, something that's clearly both the next step in their story and yet absolutely wrong. It's there for every time when you realize, just after you do something, "What have I done?"
Projects are fine. Projects are cool.
But sometimes you break the world, and then you have to live with that.
Sometimes the first time you use that Greater Creation of Sin, it's at the pinnacle of a great story, and you cause Sachiel to fall, and there's no need for a Project because it's not the goal, it's the dramatic resolution. And sometimes that leads to helping Shamshel fall, and Ramiel, and Gaghiel, and then one day, when you're talking to Leliel, and you realize how much you hate what Leliel is doing, you just go "Greater Creation of Sin."
And it doesn't fit.
It's not right.
It's broken and wrong. And you realize as the fires of Hell rise around Leliel that you have done something wrong, that you have sinned against Sin, and that there will be a price.
Maybe the price is just miraculous combat against Leliel. Maybe it's something worse.
On the tree thing, I suppose I've got a stingier interpretation of "on a larger scale" than you, which is cool. Adjusting for that, the description of the Lesser Sacrifice suggests that you could remove the "branchiness" of the branches of the World Ash, but they'd still be something else. (Pegs?) Both your tree example and your factory one just seem out-of-scale with the examples in the book (and the examples in every other edition of Nobilis ever). And they seem to raise some questions of the "Why haven't the Excrucians already done this?" form.
OTOH, interpreting it your way gives me an option to comply with the Monarda Law besides saying, "Of course you can do that … as a Project." I don't know how much benefit there is to gameplay to say, "1. You can totally drop all the branches of the Ash into Hell with a level 8 Miracle. BUT something might go wro-ong … !" or say it and wave the picture of Creepy Actual Baby around. Unless everyone thinks it would be awesome to play out the continuation in which all the branches of the Ash have crashed into Hell and a massive Actual infestation afflicts … whatever's left? In which case, go for it! But if an HG is really wielding it as a Doomsday Device - "You can do this, but don't you dare!" - that seems unlikely to make anybody happy.
Nominally, shaking the branches of the World Ash fails because someone reacts. That is, you can't just break everything, everywhere because the larger the scale of your action, the more likely the HG is to make up someone who can get in your way. In a purely Simulationist fashion.
But maybe you've found a world that nobody else knows about. And maybe it doesn't have many local Imperators or Powers, because it's just that isolated. Maybe you're powerful enough to shake that world from the branches of the World Ash. Heck, maybe you're clever, and you make your miracle so modular that you shake every insufficiently-defended world from the branches of the World Ash. What happens then?
Well, maybe nothing. Maybe that's just what happens. You have the power. You are a Power. You are sovereign in yourself.
But then maybe one day, later on, the HG is thinking about things that could happen, and remembers an act of hubris, and that those acts can spawn Actuals, and what better way to reify the guilt and horror of that act?